The invention relates to a method for measuring bending moments on a joint, wherein the joint is formed at least from two joint sections, which are flexible relative to each other about at least a bending axis, in which the joint is bent, in that a first joint section is pivoted about a pivot axis relative to a rigidly clamped second joint section.
The invention relates to the class of measurement arrangements for measuring reaction moments and forces of bending moments on individual joints. In this case, the levers are used for simplifying the holding and measuring as an extension of a joint section. Alternatively, the levers are articulated shaft sections of articulated shafts, for example, articulated shafts from drive trains of motor vehicles, when the bending moments of complete articulated shaft arrangements are measured. The articulated shaft sections are connected in an articulated manner to another articulated shaft section via a joint.
The reaction forces or reaction moments develop in the device as reactions to torques in rotating connections or in bearings or as reactions to bending moments in joints, when rotating connections or bearings are turned or joints are bent. In articulated shafts of the drive train of vehicles, the bending moment is a measure for the prevailing play in the joint arrangement. However, for example, in so-called constant-velocity joints, especially in pin universal joints, the play is an evaluation criterium for the function of the articulated shaft arrangement. Unbalanced masses around the rotational axes of the articulated shaft sections can develop due to play that is too great.
The resistance at the folding point of a joint is designated as the bending moment, which is directed opposite the bending of two articulated shaft sections connected to the joint and can be detected and thus can be measured. The bending moment is dependent on the construction of the hinged connection and is comprised, for example, from friction moments and from other resistances of the roller contact at a joint of an articulated shaft of a motor vehicle. In such joints, the value of the bending moment is set at the freedom of play of the joint. The joints are installed intentionally with pre-tensioning. Friction is intentionally set, for example, between the ends of the pin joint and the bases of the universal joint bushings. With the measurement of the bending moment, this resistance can be tested together with other resistances, for example, together with the resistances from the radial roller bearings of the universal joint bushings. For this purpose, a joint section of the articulated shaft arrangement is fixed and the other pivots about one of the axes of the pin joint by up to 90° or by a different angle of arbitrary size.
With constant-velocity joints, a hinged connection, which transmits torques and which must allow relative axial movements between the articulated shaft sections, is produced between two articulated shaft sections. For this purpose, the joints usually feature roller bodies, which are guided in raceways and on which the two joint sections roll relative to each other so that they can move in the axial direction and by means of which the joint sections are engaged with each other to transmit torque with a positive fit in the peripheral direction. The friction moments should be as small as possible in this arrangement.
Pin universal joints are hinged connections transmitting torques between two articulated shaft sections without play as much as possible in all directions. In pin universal joints, each of the articulated shaft sections is provided with a joint yoke. The two joint yokes are connected via a universal joint so that they can pivot about two joint axes and are supported usually with low friction as much as possible on the pin of the universal joint by means of roller bearings. Each of the joint axes corresponds to one of the pin joint axes, which are oriented perpendicular to each other and which cross at the center of the universal joint.
Small play in joint arrangements is important for the function of the articulated shaft. Because the constant-velocity joints should allow axial compensation, the play is positive. Positive plays are air gaps between elements supported one on the other. These plays should be as small as possible, but should also be provided to keep the bending moments small. In contrast, in pin universal joint arrangements, the pin joint and the joint yokes are mounted, as mentioned above, so that they can move relative to each other, without play, and with pre-tensioning. In order to guarantee freedom of play, the elements are preferably mounted relative to each other with negative play, that is, with pre-tensioning. A measure for the freedom of play or the measure for the pre-tensioning, with which the joint yokes and the pin joint are to be mounted or are assembled with each other is the bending moment, with which the pre-tensioned joint can bend about the respective joint axis.
DE 39 22 194 C1 describes a method and a device of the most general form for measuring bending moments in pin universal joint arrangements. The device is formed by a holder, with which an articulated shaft section is held stationary. The joint yoke of this joint section is oriented in the device so that the other articulated shaft section is driven by the pivot drive so that it can pivot about the joint axes of the pin joint. A bending rod, whose fibers of the outer skin are elongated or compressed as a function of bending direction and resistance of the joint, is arranged between the pivoting joint section and the pivot drive. Expansion measurement strips, with which the expansion of the fibers is detected and converted into corresponding electrical voltage magnitudes, are arranged on the outer skin. The pivot drive is connected in an articulated way to a radial guide and then via a ball-and-socket joint to the bending rod. The radial guidance can pivot with a pivoting angle of 90° about the rotational axis of the articulated shaft arrangement in the sense of rotation by means of the pivot drive.
With the method described in DE 39 22 194 C1, in the device counter-acting bending moments about the two joint axes when the moving joint section bends relative to the rigid joint section are measured. For this purpose, a radial guidance is pivoted about the rotational axis on an arc by 90° in the sense of rotation by means of the pivot drive. Here, the counteracting bending moments on the joint axes are first detected in the form of tension magnitudes on the expansion measurement strips of the bending rod. These tension magnitudes are proportional to the bending moments, are recorded, and are selectively converted and displayed legibly in a display device.
DE 41 02 278 A1 shows and describes a device for measuring forces and moments in articulated shaft arrangements with constant-velocity joints. This device has a stationary receptacle, in which one of the joint sections is held rigidly. The other articulated shaft section can pivot relative to the fixed articulated shaft section by the joint. A so-called force-measuring device for force-path measurement, in which the pivoting articulated shaft section is held, is arranged on the pivot axis between the contact of the pivot drive and the joint. The bending moments are detected at force measurement sensors as deflections (path due to force), which are caused in the device by reaction forces to the moments on the bearing.
A portion of the weight of the first articulated shaft section is supported in the receptacle. Another portion of the weight of the first articulated shaft section is supported in the joint. Articulated shaft sections are heavy, so that the portions of weight to be supported are relatively high. Depending on the center of gravity of the first articulated shaft section, on the distance of the bending axis to the contact in the receptacle, and on the lever of the center of gravity to the bending axis, additional forces and moments are generated in the joint. The influence of this portion of the weight to the bending moments in the joint can therefore be relatively high and higher than the actual original bending moments appearing in the bearing. The measurement results can be falsified. Therefore, every new articulated shaft installed into the device must be calibrated in a complicated process before the beginning of the measurements of bending moments. Due to this calibration, however, the influence of the weight on the bending moment cannot be excluded, because the magnitude of the influence is scarcely to be determined and alternates from measurement arrangement to measurement arrangement. Consequently, the measurement results are faulty.
In the arrangement from DE 41 02 278 A1, an articulated shaft section is held in a receptacle, which is supported in the force measurement device so that it can move radially and axially by means of elastic means on carriers. Carriers are fixed in place, for example, on a base plate of the measurement device. The elastic elements should counteract the axial and radial movements and as much as possible have no restoring forces. Force measurement sensors are arranged between the suspended receptacle moving radially and axially and the non-moving carriers.
With force measurement sensors, usually the forces acting on the sensor are not measured directly. These sensors react to the displacement of objects from a starting position with displacements of sensor elements or through their deformation. The displacements and deformation result from forces or from moments. The reactions in the device are first displacements against defined resistances and then the displacement or deformation of sensor elements. In one evaluation device, the conversion of signals due to deformation into force measurement values is finally performed.
With force sensors, usually compression, shear, and tension forces are all measured. Most force sensors work with at least one spring-elastic body, whose elastic deformation is measured, or they react in a different way, for example, to changes in position using moving elements. Examples for such sensors are tension or compression rods or bending beams or membrane force sensors with expanding measurement strips.
Alternative force measurement sensors are, for example, piezoelectric force sensors that react to pressure. In a piezoceramic element, a voltage that is proportional to the force is generated due to the force. This voltage can be measured. The use of any suitable force sensor, for example, force sensors with electro-magnetic compensation or other force sensors with distance sensors and current regulation, is also conceivable.
In DE 41 02 278 A1, a measurement device is described, in which the receptacle is supported on elastic elements on a moving carrier. The elastic elements are elastically flexible like a hinge only in the pivoting directions, in which the articulated shaft section is pivoted for measuring bending moments. The bending moments are detected at sensors as deflections of the receptacle from an origin or position. The deflections are caused by reaction forces to the moments on the bearing. Sensors, which receive the deflection of the articulated shaft section, are each arranged between fixed carriers and the moving articulated shaft section. Dimensional and position deviations, for example, alignment errors between the rotational axes of the lever and the longitudinal axis of the receptacle in the device, can have the result that the receptacle assumes a position, which does not correspond to the rest position of receptacle before the measurements, because the receptacle is mounted in a spring-elastic way and avoids constraining movements due to the deviations. Because the sensors of the class-forming state of the art are in constant contact with the receptacle, these undesired displacements are already detected in the rest position as restoring moments or forces and falsify the actual measurement values. Therefore, their influence must be removed through calibration before the beginning of each new measurement.
As described in DE 41 02 278 A1, the measurement values are influenced by a coupling equalizing axial movements and by restoring moments of the elastic elements. Furthermore, the accuracy of the measurement results is dependent on the type and construction and engagement of the pivoting drive. The pivoting drive must be sensitive and steady, in order to avoid negative effects on the measurement results.